French patent application No. 78-14,494 and the article "Mesure du debit aortique par echo Doppler et sonde intraoesophagienne" (Journal Francais d'Echographie, October 1983) describe a medical procedure whereby one inserts a listening probe into a patient's esophagus to monitor aortic flow. The probe has a flexible tube provided at one end with an inflatable rubber balloon inside of which is provided at least one sensor, typically an ultrasonic transducer. Feed and output cables extend inside the tube from the sensor to the outside. Once inserted to the proper level in the esophagus the probe can be filled up to swell it and cause it to tightly fill the esophagus so as to form a good sound-transmitting connection with the surrounding body structures, in particular the aorta.
In order to orient the sensor so it is facing the right way once the probe is installed, it is carried at one end of a nontwist cable that extends back along the tube and that is attached to operating mechanism so its rear end can be twisted, thereby rotating the sensor carried on its opposite front end inside the balloon. The cable fits somewhat loosely in the tube so that it can rotate fairly freely therein.
Such a setup has several main disadvantages. First of all it is fairly difficult to insert the probe, which typically must enter via the patient's nose as the conditions normally checked with such a probe are only performed on gravely sick patients who already have mouth tubes. The probe end of the assembly is fairly rigid and hard, being formed by the support and the sensor it carries covered by the latex balloon, so such insertion is very difficult and often fairly painful. In addition the unit is fairly complex so that the probe itself is an expensive item to manufacture, entailing molding and electronics technology at high tolerances.
The latex forming the balloon and the tube must be very thin so that it poses minimal interference to the taking of accurate measurements. As a result it is fairly fragile and after some use, normally more than ten times, it is worn out. Similarly, the tube is worn by friction against the cable inside itself. Since the entire assembly is thus rendered useless, the result is that the cost per use of such a probe is very high, as the high cost of manufacture cannot be recovered over a meaningful long service life.